Transparent shadow not appearing in viewer node
Not sure what you're trying to accomplish here.
Right now your compositing nodes will display nothing because there's nothing fed in the input color. So basically you ask Blender to display "nothing" according to an alpha map.

Besides, I'm pretty sure you want to display this shadow on the floor beneath your model. In which case, what you want to do is either use a material on that flood to display shadows where you want and not use compositing at all.

Something like that:


Compositing is meant to make changes on the rendered picture. May it be color corrections, deformations, ... But it's really on the rendered picture level, not on what will be rendered. Think of it like editing pictures with Photoshop, but using nodes.